Treating subsurface formations

ABSTRACT

Shaley earth formations are drilled or otherwise treated with reduced difficulty through the use of water-in-oil invert emulsion fluids wherein the aqueous phases of the emulsions possess particular water vapor pressures relative to the formations which they contact. The aqueous vapor pressure of an oil-base fluid containing dispersed water is controlled to prevent damage to water-sensitive shale formations by monitoring the vapor pressure of the aqueous phase of the fluid and maintaining a vapor pressure depressant in the aqueous phase in a concentration sufficient to substantially prevent the migration of water from the fluid to the formations. The aqueous vapor pressure of an earth formation is determined. A method and apparatus are disclosed for determining the compatibility of a well fluid with a water-sensitive subsurface formation wherein a substantially unaltered sample of the formation is immersed in the fluid and the direction and extent of water migration between the well fluid and the sample are logged. Improved water-in-oil invert emulsion fluid compositions for drilling and other oil field uses are obtained wherein the aqueous phases of the emulsions possess particular water vapor pressures relative to the formations which they contact.

nited States Patent Chenevert [54] TREATING SUBSURFACE FORMATIONS [72]Inventor: Martin E. Chenevert, Houston,

Tex. 77027 [73] Esso Production Research Company [22] Filed: March 16,1970 [21] Appl. No.: 19,574

Related US. Application Data [63] Continuation-impart of Ser. No.726,693, May

6, 1968, abandoned, which is a continuation-inpart of Ser. Nos. 675,490,Oct. 16, 1967, abandoned, and Ser. No. 699,255, Jan. 19, I968,

abandoned.

[52] US. Cl. ..175/50, 175/65, 175/66, 252/85 P, 166/250 [51] Int. Cl...E2lb 21/04 [58] Field of Search.....252/8.5 M, 8.5 P; 175/40, 50,175/65, 66, 72; 166/250, 275

Moore, John E., How to Combat Swelling Clays, in Petroleum Engineer,Mar. 1960, pp. B- 78, 90, 94, 95,

[ 1 Sept. 5, 1972 Primary Examiner-Stephen J. Novosad Attorney-James A.Reilly, John B. Davidson, Lewis I-l. Eatherton, James E. Gilchrist,Robert L. Graham and James E. Reed [57] ABSTRACT Shaley earth formationsare drilled or otherwise treated with reduced difficulty through the useof water-in-oil invert emulsion fluids wherein the aqueous phases of theemulsions possess particular water vapor pressures relative to theformations which they contact. The aqueous vapor pressure of an oil-basefluid containing dispersed water is controlled to prevent damage towater-sensitive shale formations by monitoring the vapor pressure of theaqueous phase of the fluid and maintaining a vapor pressure depressantin the aqueous phase in a concentration sufficient to substantiallyprevent the migration of water from the fluid to the formations. Theaqueous vapor pressure of an earth formation is determined. A method andapparatus are disclosed for determining the compatibility of a wellfluid with a water-sensitive subsurface formation wherein asubstantially unaltered sample of the formation is immersed in the fluidand the direction and extent of water migration between the well fluidand the sample are logged. Improved water-in-oil invert emulsion fluidcompositions for drilling and other oil field uses are obtained whereinthe aqueous phases of the emulsions possess particular water vaporpressures relative to the formations which they contact.

35 Claims, 8 Drawing Figures PATENTEDsn 51972 SHEET 3 OF 5 BALANCEDACTIVITY ACTIVITY OF MUD, u

M O O vINVENTOR. MARTIN E. CHENEVERT BY A ATTORNEY WATER CONTENT, WEIGHTDEPTH, FEET PAIENIEBSEP' 5 I912 8.888.851

SHEET '4 BF 5 A I 0 0| 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 L0 RELATIVE VAPORPRESSURE, p/p

FIG. 6

INVENTOR. [8,000 MARTIN E. CHENEVERT WATER CONTENT, WEIGHT "/8 ATTORNEYTREATING SUBSURFACE FORMATIONS CROSS REFERENCES TO RELATED APPLICATIONSnow abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isdirected primarily to the drilling of wells through water-sensitiveearth formations. More particularly, the invention is concerned withmethods for formulating drilling fluids, methods for drilling, anddrilling fluid compositions which enable formations to be drilled with aminimum of difficulty. The invention also pertains, in general, tomethods for designing and applying fluids which find use in contact withshaley earth formations, and fluid compositions for such use.

2. Description of the Prior Art The problems encountered in drillingthrough shales and similar water-sensitive formations are of longstanding in the petroleum industry. Such problems are particularly acutein deep formations containing hard shales but are also troublesome inshallower formations. The nature and extent of the difficultiesencountered generally depend in part upon the characteristics of theparticular water-sensitive formations to be penetrated. Geologicallyspeaking, such formations normally contain fine-grained argillaceousmaterials which in a compressed state have very low permeabilities,generally less than 0.001 millidarcy. Since such materials may have beendeposited under widely dif ferent circumstances and subsequentlysubjected to different temperature and pressure conditions,water-sensitive formations may have mineral compositions and watercontents which vary considerably.

Water-sensitive shales are broadly categorized in the petroleum industryas either soft shales or hard shales. Soft shales are generallyconsidered to include the plastic, gumbo, wet, hydratable, and swellingshales which dissolve or disperse readily in water. Soft shales usuallyoccur in young stratigraphic sequences and in the shallower formationswithin any stratigraphic sequence. Hard shales, on the other hand, arehard and are substantially insoluble and nondispersible in aqueousmedia. They generally contain much less water and montmorillonite thando soft shales. Hard shales generally occur in older, deeper formations.

Soft shales and hard shales both present serious problems duringdrilling operations. If soft shales are drilled with water or water-basefluids, for example, they may readily disperse or dissolve in thecirculating fluid to form a troublesome plastic mass. This dispersionhas been generally attributed to the presence of montmorillonite in theshales. I-Iard shales, on the other band, do not normally disperse inthe fluid but nevertheless quickly lose their strength and break andslough. Until recently there has been no satisfactory or acceptedexplanation for the failure of hard shales. The extremelylow'permeabilities of these shales have led observers to theorize thatdrilling fluids can penetrate only along the bedding planes and that thefailures which occur are due to such penetration. It has also beengenerally assumed that hard shales are nonswelling in the presence ofwater.

Difficulties in drilling through gumbo and other soft shales have beenalleviated to some extent in the past by the use of water-in-oil invertemulsion muds and other oil-base fluids. The use of invert emulsion muds0 prepared with saturated sodium chloride solutions or containingmoderate amounts of calcium chloride, normally less than 250,000 partsper million, in the water phase of the emulsions has also provided someimprovement. The results obtained with such fluids, however, havegenerally been inconsistent and unpredictable.

Oil-base muds, including water-in-oil invert emulsion muds, have alsobeen used for drilling hard shales. In most instances such use hasprovided little, if any, improvement over the use of water-base muds.Serious mud losses, formation damage, and increased operating expenseshave often been incurred. Although increasing the weight of the drillingfluid and drilling more slowly has often helped, efforts to overcomedifficulties encountered with hard shales have in the past been largelyunsuccessful.

SUMMARY OF THE INVENTION This invention provides means for alleviatingproblems normally encountered when shales and similar water-sensitiveargillaceous earth formations are contacted with aqueous fluids. Theinvention greatly extends the application of oil-base drilling fluidsfor drilling water-sensitive formations and is especially useful indrilling or treating hard shale formations such as the Midway and Dextershales in East Texas and West Louisiana, the Wolfcamp shale in WestTexas, the Springer and Atoka shales in Oklahoma, and similar hardshales elsewhere.

In accordance with the invention, it has now been found that shales,shaley sands, and similar argillaceous formations, in spite of theirextremely low permeability, possess a strong attraction for water andare capable of withdrawing water from water-in-oil emulsions and otherfluids with which they come in contact. This sensitivity to water isevidenced by dimensional changes in response to the absorption ordesorption of water. These changes, although sometimes very slight,contribute materially to formation failure. It has been found that therate at which such a formation withdraws water from a particular aqueousfluid is a quantitive measure of the degree of water sensitivity of theformation in the presence of that fluid. This rate and hence the watersensitivity of the formation can be assessed by at least partiallyimmersing a substantially unaltered sample of the formation in the fluidand measuring the changes in dimensions, weight, or other properties ofthe sample, directly or indirectly, over a selected period. A preferredmethod of measuring the water sensitivity of the formation is to measurethe deformation rate, whether visible or subvisible, of a formationsample in the presence of the fluid.

Although the mechanisms responsible for the transfer of water betweenthe emulsion fluid and the argillaceous shale with which the emulsionfluid comes in contact are evidently complex and are not fullyunderstood, experience has shown that water transfer from the emulsionfluid to the shale will normally occur if the vapor pressure of theaqueous phase of the fluid is greater than the vapor pressure of theformation. Measurement of vapor pressures thus provides a convenienttechnique for the evaluation of emulsion fluids. Aqueous vapor pressureis directly proportional to the activity of water and hence watertransfer will normally occur from emulsion to shale when the activity ofthe water contained within the aqueous phase of the emulsion exceedsthat of water contained within the shale. It is important to note thatthe aqueous vapor pressure of the formation normally differs from thevapor pressure of the water or brine contained within the formation. Itappears that certain electrical or absorptive forces associated with thematrix or composition of the formation itself greatly decrease the vaporpressure which the water contained therein would otherwise be expectedto have. Measurement of the aqueous vapor pressure of the formation,which characterizes the activity of the formation water, is therefore animportant aspect of the invention.

Two general methods for designing oil-base drilling fluids in accordancewith the invention are disclosed. Both involve the addition of vaporpressure depressants to the aqueous phase of the emulsion fluid inamounts sufficient to eliminate or to retard transfer of water from thedrilling fluid to the argillaceous formation. The first method is adirect simulation of the interaction of the fluid and thewater-sensitive formation. A water vapor pressure depressant ispreferable first dissolved in the aqueous phase of the emulsion drillingfluid. The rate of water transfer between this fluid and the formationis then quantitatively determined by immersing a sample of the formationin substantially its natural state in the fluid and determining the rateof deformation. The concentration of the water vapor depressant can thenbe increased and additional samples tested until a concentration thatreduces the rate of deformation to substantially zero is found. Adeformation rate that for all practical purposes approaches zeroindicates that the fluid can be used with little likelihood of damagingthe formation.

A second method for designing drilling fluids requires that the aqueousvapor pressure of the argillaceous shale formation first be determined.This can be done by exposing formation samples to atmospheres abovedifferent saturated salt solutions having known water vapor pressuresuntil equilibrium is reached. By observing the weight change of thesample resulting from water migration, the vapor pressure of anatmosphere that would result in no weight change is determined. Thisvalue represents the formation vapor pressure. After thus determiningthe vapor pressure of the shale formation, an emulsion fluid having anaqueous vapor pressure substantially equal to that of the formation canbe prepared. Such a fluid can be used to drill the water-sensitiveformation with little likelihood of the hole sloughing.

It is still a further aspect of the invention to provide a method andapparatus to monitor invert emulsion drilling fluids at the well siteduring drilling operations so that the fluid composition can be changedto compensate for drilling fluid contaminants and the like encounteredin the borehole. Another aspect of the invention is to provide improvedwater-in-oil emulsion drilling fluid compositions. While oil-base fluidsand invert emulsion fluids, which are both water-in-oil emulsion fluids,are sometimes distinguished on the basis of water content, all threeterms are treated as synonymous herein. It is still another aspect ofthe invention to provide methods for designing improved coring fluids,treating fluids, fracturing fluids, displacement fluids, and the likewhich contact water-sensitive formations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically depicts anelevation view of a displacement transducer instrumented with straingauges suitable for performing the simulation test method of theinvention.

FIG. 2 is a schematic plan view of the apparatus of FIG. 1.

FIG. 3 is a schematic diagram of an electrical circuit that can be usedwith the apparatus of FIG. 1.

FIG. 4 graphically illustrates unit elongation versus log time datarecorded while testing a hard shale in accordance with the simulationtest method of the invention.

. FIG. 5 graphically illustrates the rate of deformation exhibited by anumber of samples of an argillaceous shale formation contacted bywater-in-oil emulsion drilling fluids having different aqueousactivities.

FIG. 6 shows the water vapor pressure (P), relative to the vaporpressure of pure water (P exhibited by a West Texas hard shale at 25 Cfor various water contents within the shale.

FIG. 7 is a correlation showing the average variation in the watercontent of a shale in terms of depth of burial within the earth.

FIG. 8 is a correlation showing the average vapor pressure (P) of twohard shales and one soft shale relative to the vapor pressure of purewater (P,,) at 25 C for different shale water contents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I. Design of Drilling Fluids A.The Simulation Test Method 1. Nature of the Simulation Test Method Thesimulation test is based on the discovery that the rate at which shalesand other argillaceous formations absorb water from a particular aqueousfluid is a quantitative measure of the degree of water sensitivity ofthe formation in the presence of the fluid. The test is performed by atleast partially immersing a sample of the formation which is insubstantially its natural state of hydration in the drilling fluid ofinterest and determining the rate of water absorption.

One method for determining the water absorbed is bychange of weight ofthe sample. The sample is weighed initially and its change in weightobserved over a period of time. Any change in weight which occurs isattributable to the migration of water. Weight measurements can beobtained while the sample is immersed by suspending it in the drillingfluid and periodically recording the suspended weight. In lieu of this,the sample may be withdrawn from the fluid after a fixed period of time,cleaned, and then weighed. Another method recognizes that theresistivity of the sample will decrease as it absorbs water and utilizeschanges in resistivity to measure the amount of water absorbed. Stillother methods are based on the measurement of changes in sonic velocity,compressive strength, and other physical properties which vary withwater content to indicate the rate of absorption.

The preferred method of measuring absorption is to log the rate ofchange in dimensions of a shale sample while it is immersed in thedrilling fluid. This gives a direct measurement of the deformation oftheshale due to the drilling fluid and thus provides a quantitativemeasurement of the rate of water absorption. A wide variety of devicesfor recording changes in dimensions may be used, including micrometers,optical equipment, dial displacement indicators, and the like. Thepreferred apparatus, however, is a displacement transducer instrumentedwith strain gauges.

2. The Displacement Transducer Apparatus FIGS. 1 and 2 illustrate aresistance strain gauge displacement transducer suitable for measuringthe change in dimensions of a sample of shale or similar material. Thisapparatus includes a rectangular base from which a substantiallycylindrical column 12 extends vertically. A series of beveled teeth onthe upper portion of column 12 form rack 14.

Cantilever deflection beam 22 engages rack 14. The outermost end of thedeflection beam extends downwardly in an L-shape terminating in afrusto-conical end terminus 24. Contactor shoe 21 is mounted on endterminus 24. The innermost end of the deflection beam 22 contains agenerally oval aperture 23, one end of which forms a yoke that fits overupright column 12 and forms a slidable support with the column. Shaft 18passes through deflection beam 22 at the other end of aperture 23. Knobs16 are mounted on the ends of shaft 18. Pinion is supported on the shaft18 in a position corresponding to the middle of the aperture tocooperate with rack 14. Upper strain gauges 25 and 26 are mounted on theupper side of deflection beam 22. Lower strain gauges 27 and 28 arepositioned on the other side of the beam.

A cylindrical pedestal 30 extends from a rectangular base 10 underneathcontactor shoe 21. The upper surface 31 ofthe cylindrical pedestal issmooth and forms a bearing surface underneath shale sample 34.Cylindrical cup 29 slides upon cylindrical pedestal 30. Sealing member32 is mounted between the cup and the pedestal to prevent the leakage offluids.

FIG. 3 illustrates an electrical circuit suitable for use with thestrain gauge displacement transducer apparatus. A four-resistorelectrical bridge in which strain gauges 25, 26, 27, and 28 form theresistors is shown. At least four resistors are generally used to obtainincreased amplitude and inherent temperature compensation. Variableresistor 32 is placed in the circuit to balance the bridge prior tostrain measurements. Voltage source 35 creates a difference in potentialacross resistor 32 and across the bridge between contacts 36 and 37,causing direct current to flow through resistor 32 and the legs of thebridge formed by resistors 25 and 27, and 26 and 28, respectively.Voltage is measured between terminals 40 and 42 by voltmeter 44. In lieuof this, a suitable strain indicator, such as Model P-350 sold by TheBudd Company, Phoenixville, Pennsylvania, could be used. Switch 46 isused to turn the strain gauge transducer on and off. Although therelatively simple strain gauge circuit illustrated is suitable, othercircuits such as those illustrated in M. Hetenyis book, Handbook ofExperimental Stress Analysis, John Wiley & Sons, Inc., New York, NewYork (1950) could readilybe adopted.

Prior to using the strain gauge transducer, it must be calibrated todetermine the relationship between observed voltages and displacement.This can be done by first zeroing the voltmeter, as is discussed below,and then placing successively larger or smaller articles of known lengthbetween contactor shoe 21 and cylindrical pedestal 30 and observing thevoltages. From these data a constant that relates voltage anddisplacement can be obtained.

To use this equipment to analyze the compatibility of a drilling fluidand a particular shale, a sample of the shale should be placed onsurface 31 of cylindrical pedestal 30. Deflection beam 22 is thenlowered by turning knob 16. This rotates shaft 18 on which pinion 20 ismounted. Pinion 20 cooperates with rack 14 to convert the rotationalmovement of the knob 16 into a downward translational movement of beam22. The beam should be lowered until contactor shoe 21 engages the shalesample 34 and holds it firmly in place on surface 21 of the pedestal 30.

With the shale sample thus mounted, the strain gauges electrical circuitshould be balanced. Voltage source 35 is energized by closing switch 46,causing current to flow through variable resistor 32 and both sides ofthe resistance bridge. The bridge is balanced by adjusting variableresistor 32 until voltmeter 44 is zeroed. Once the bridge has beenbalanced, the voltage readings will indicate deformation. Cylindricalcup 29 is then raised to its uppermost position so that the upper edgesof the cup are above the top of sample 34. Sufficient drilling fluid tocover the sample is then poured into the cup held between contactor shoe21 and surface 31 of cylindrical pedestal 30.

Once the drilling fluid contacts the sample mounted within the straingauges, the sample will begin to absorb water and expand if it isincompatible with the fluid. Expansion of the sample will forcecontactor shoe 2] upward, deflecting beam 22. Deflection of the beamresults in deformation of the strain gauges and produces an imbalance involtage readings across the bridge. If the fluid absorbs water from thesample, the sample will generally exhibit shrinkage. Such shrinkage alsonormally produces an imbalance in voltage readings across the bridge.However, these voltages will have an opposite sign from those causedbyswelling.

Several voltage readings should be taken at various times after thesample has been immersed in the drilling fluid. The voltage readings areproportional to the displacement of the sample between contactor shoe 21and pedestal 30. The relationship between displacement and time can bedetermined from the calibration constant and used to determine the rateat which this sample will absorb water from the particular drillingfluid. When comparing data, it is useful to normalize the displacementdata by dividing each reading by the sample length. The normalized datais then referred to as strain. The rate so determined is indicative ofthe degree of compatibility between the water-sensitive formation andthe drilling fluid.

3. Selection and Preparation of Formation Samples The determination ofthe water sensitivity of a subsurface formation in the presence of aparticular drilling fluid in accordance with the invention is normallycarried out with a sample of the formation having substantially its insitu composition. Exposure to high temperatures and other treatment thatmay alter the composition should be avoided. It is preferred that thissample be in substantially its natural state of hydration so that itssurface absorption behavior will approximate in situ absorptionbehavior. Laboratory tests performed at reservoir conditions oftemperature and pressure, when compared with absorption tests conductedunder atmospheric conditions, indicate that atmospheric tests aresufficiently accurate for most practical purposes.

The formation samples utilized may be preserved core samples from thesubject well or from a nearby well that penetrates the same formation.Such preserved samples are particularly representative when the coringfluid used inhibits absorption of water by the water-sensitiveformation. Fragments of the formation entrained by the drilling fluidand carried to the surface can also be used. Since a water-sensitiveformation will begin hydration as soon as it is contacted with awatercontaining drilling fluid, it is preferable that such fragments berecovered as early as possible after initial contact of the rock by thefluid. Hence, the depth of the formation of interest should be estimatedand samples from the earliest returns from drilling the formation shouldbe secured for the test. The use of an oil-base drilling fluid treatedin accordance with the invention generallysimplifies the recovery ofsamples in substantially their natural state of hydration.

Where severe hydration of the formation has occurred, the samplesobtained should be restored to their natural state of hydration.Hydration is not always encountered when the drilling fluid is a treatedoil-base fluid and is generally more severe where a water-base fluid isused to drill a highly water-sensitive shale. Restoration to asubstantially natural state can be accomplished by baking the samples ata temperature slightly above 100 C until sample density corresponds withtypical shale density for this formation and depth of burial. Sampledensity can be rapidly determined by means of a graduated density liquidcolumn, the mercury pump pressure chamber method, or other suitabletechniques. Correlations of shale density versus depth of burial areavailable in the literature for various formations and are typified bythose published by K. F. Dallmus in his study Mechanics of BasinEvolution and Its Relation to the Habitat of Oil in the Basin, Habitatof Oil A Symposium, Tulsa, Amer. Assoc. Petrol. Geol., 1958, p. 883-931.It is important that temperature not greatly exceed 100 C sinceexcessive temperatures may result in substantial changes incharacteristics of the sample.

4. Drilling Fluid Design Use of the method and apparatus of theinvention to formulate an oil-base drilling fluid that will prevent orminimize absorption and thus promote borehole stability is based in parton the observation that an oil-base or water-in-oil emulsion mud havingan aqueous vapor pressure substantially equal to or less than that ofthe troublesome water-sensitive formation will prevent absorption ofwater by the formation. Samples of the water-sensitive formation insubstantially their natural state should be used, as indicated above.Several of these samples are preferably immersed in a correspondingnumber of different oil-base drilling fluids having different aqueousvapor pressures and straintime data are obtained for eachfluid-formation combination. This procedure can be greatly expedited byusing a number of strain gauge displacement transducers.

A series of water-in-oil emulsions or other oil-base muds havingdifferent aqueous vapor pressures can be prepared by adding variousconcentrations of inorganic salts such as NaCl or CaCl to the mud. Anumber of other vapor pressure depressants are discussed herein inconnection with the method of determining the vapor pressure of an earthformation. Suitable vapor pressure depressants are not limited to theseor similar inorganic salts, however. Any solute introduced into theaqueous phase will reduce the aqueous vapor pressure.

FIG. 4 illustrates strain-time data obtained in accordance with theinvention for the hard, argillaceous Wolfcamp shale. Fluid A is water,and the high rate of absorption for this fluid is typical of a verycompatible fluid. Fluids B, C, D, and E are water-in-oil invertemulsions containing in the aqueous phase, as vapor pressuredepressants, 130,000-ppm NaCl, 200,000-ppm NaCl, 270,000-ppm NaCl, and450,000-ppm CaCl respectively. Curves B, C, and D illustrate thereduction in absorption that occurs as the concentration of the aqueousvapor pressure depressant is increased and the aqueous vapor pressure ofthe fluid approaches that of the formation. Curve E illustrates behaviorcharacteristic of a water-in-oil emulsion mud with an aqueous vaporpressure that has been reduced below that of the water-sensitiveformation. Instead of swelling, the sh ale sample shrinks, indicatingthat water is being desorbed from the shale sample. The use of adrilling fluid with a composition similar to that of mud E wouldtherefore prevent absorption of water by the shale. Generally, however,there is little incentive in attempting to dehydrate a water-sensitiveformation and therefore such a fluid would normally be considered tocontain an excessive amount of vapor pressure depressant. In most casesit would be more economical to reduce the concentration of CaCl in FluidE so that its strain-logtime curve would more closely approach the zerostrain line than to use a mud such as Fluid E.

FIG. 5 illustrates graphically the rates of deformation of a series ofshale samples exposed to invert muds having varying aqueous activities(relative vapor pressures). The shale formation on which the tests wererun had an aqueous activity of 0.7. Each test involved immersing a shalesample in an invert mud having a known aqueous activity for a period of10 hours, measuring the strain, and then computing the average rate ofstrain of the sample over this time period. It will be noted that shalesamples exposed to muds having aq ueous activities higher than 0.7swelled and that the observed rate of swelling increased as thedifference in aqueous activity between the mud and the sample increased.Samples contacted with muds having aqueous activities lower than 0.7shrank. Again, however, the rate of deformation increased in relation tothe activity difference. These data demonstrate that when a differencein activity exists, water will flow either from the emulsion mud to theshale or from the shale to the mud.

The former causes swelling of the water-sensitive subterraneanformation, leading to its sloughing into the wellbore; the latterincreases the water content and thus viscosity of the drilling fluid,necessitating frequent additions of oil, salt and other materialsrequired to maintain the drilling fluid. However, when the activity ofthe mud is substantially equal to that of the shale formation beingdrilled, a unique relationship exists. So long as this balancedcondition is maintained, there is substantially no migration of water ineither direction. Thus, it is especially desirable to maintain theaqueous activity of the mud about equal to that of the shale and therebyboth eliminate sloughing of the borehole and obviate the addition ofsalt, oil or other materials to the mud normally required when it iscontaminated by water.

Although the simulation test has been discussed in relation towater-in-oil emulsion drilling fluids the utility of the simulation testis not limited to this type of drilling fluid. The simulation testmethod and apparatus can be used to determine the compatibility of anydrilling fluid with a water-sensitive formation and can be employed toselect the most compatible drilling fluid from any group of drillingfluids. The method and ap paratus can also be used to determine whetheror not a particular formation is water-sensitive and to select fluidsfor use in secondary recovery, well stimulation, or other welloperations, as is more fully discussed subsequently herein.

B. The Formation Vapor Pressure Test Method 1. The Method of Determiningthe Vapor Pressure of an Earth Formation The aqueous vapor pressure of ashale or other water-containing earth formation can be determined bysubjecting a sample of the formation to air of a constant known humidityfor a period of time sufficient for moisture within the shale to reachequilibrium with the moisture in the air. It will normally be difficultto preselect a humidity condition such that the natural water content ofthe shale will be in equilibrium with this condition of humidity. So,generally speaking, several different humidity conditions must be usedto obtain a range of water contents within the sample which will spanthe in situ water content of the formation within the earth.

A very convenient procedure for exposing samples of a given formation toair of different humidities is to suspend or place the sample in asealed container in an atmosphere of air above a saturated aqueoussolution of a solute which contains an excess of undissolved solute.Thus, it is known that the relative humidity of the enclosed space abovesuch a solution where the sample has been placed will remainsubstantially constant at a given temperature-conveniently roomtemperature (25 C). An article containing an explanation of thisprinciple and also listing a number of saturated solutions and solutesis contained in Ecology: Vol. 41; No. 1; pp.232-237 (January, 1960).Typically, a series of several different saturated solutions can beprepared, and one or more samples of a given shale or other formationcan be exposed to an enclosed atmosphere above each of these samples fora sufficient period of time for equilibrium to occur. Completeequilibrium will normally take about 1 or 2 weeks, but substantialequilibrium can normally be attained in about 1 or 2 days.

Saturated solution of Relative humidity (96) at 25C ZnCl,. 1% H 0 10.0CaCl, 6H O 29.5 Ca(NO;,) 4H,O 50.5 NH C| KNQ', 71.2 (M1, so, 80.0 NaTamale 92.0 KH,PO.| 96.0 K,Cr,0 98.0

Many of these salts, incidentally, may themselves be used within theaqueous phase of invert emulsions for the purpose of establishing thevapor pressure of that phase. If vapor pressures less than thatobtainable for a saturated calcium chloride solution are desired,solutions of ZnCl 1 /2 H O; LiCl H O; ZnBr- LiBr 21-1 0; potassiumhydroxide or other stronger vapor pressure depressant may be employed.The depressant, of course, must be compatible with the invert emulsionof interest; and such compatibility should be tested prior to actualuse.

After a formation sample has reached equilibrium with a particularatmosphere of known relative humidity, the sample should be withdrawnfrom the atmosphere and its water content promptly determined. A simpleprocedure for determining its water content is to weigh the equilibratedsample, and then repeat the weighing after the sample has been dried atabout 105 C for a period of 12 to 24 hours. The loss in weight of thesample is a direct measure of the equilibrated water content of thesample. The vapor pressure of the sample for this water content is thevapor pressure of water at room temperature (or the temperature of theequilibrium condition) multiplied by the percent relative humidity ofthe air in equilibrium with the sample.

After the vapor pressures and water contents of a given sample or set ofsamples have been determined, these values can be recorded on a suitablechart or other record medium and intermediate values can be determinedfrom the resulting correlation. Thus, FIG. 6 of the drawing shows twocorrelations (A for absorption conditions, and D for desorptionconditions) obtained by subjecting samples of a West Texas hard shale toeight different conditions of relative humidity ranging from 10 percentrelative humidity to 98 percent relative humidity at a temperature of 25C. These curves also apply for temperatures at least as high as 100 C.As can be seen, slightly different correlations were obtained for testsin which water was desorbed from the shale samples as compared withtests in which water was absorbed by the shale samples. The shale samplein this instance had an in situ water content of 2.22 weight percent asdetermined by analyzing a small central portion of a core out directlyfromthe formation under conditions such that the water content of mostof the core was undisturbed. From FIG. 6, it is apparent that thisformation has a water vapor pressure (or formation vapor pressure)relative to the vapor pressure of pure water of between about 0.71 and0.81.

Another convenient method for determining the aqueous vapor pressure ofa water-sensitive formation is to place a sample that is representativeof the subsurface formation in a sealed container until it reachesequilibrium with the enclosed atmosphere. A direct measurement of therelative humidity of the formation sample can then be made. This methodis also useful for determining the aqueous vapor pressure of awatercontaining oil-base drilling fluid. Apparatus for measuringrelative humidity is widely available. Typical of such apparatus is theCatalog No. 2200 ELECTRO- HYGROMETER that is sold by Lab-LineInstrument, lnc., Melrose Park, Ill.

2. Selection and Preparation of Samples As indicated earlier, the use ofthis invention in designing well fluids should be preceded by adetermination of the vapor pressure characteristics of the portions ofthe zones or formations which the emulsion fluid will contact. In thecase of a drilling operation, as pointed out earlier in the discussionof the simulation test, a sample of the formation of interest should beobtained so that its vapor pressure can be determined. If a sample ofthe formation is not obtainable directly from the well being drilled,then an effort should be made to obtain a sample from a nearby well. Itis also possible, however, to collect and use cuttings from the wellwhich is being drilled.

Again, the preferred type of formation sample to obtain and study is asample from the central portion of a core which has been cut from theformation under conditions suitable to preserve the natural conditionsof the core as much as possible. If such a sample is available, areasonably accurate determination can be made of the amount of in situwater contained in the core. If such a core cannot be obtained, theformations water content can be estimated from FIG. 7. FIG. 7 is acorrelation showing how the water content of many shaley formationswithin the earth vary, on the average, with increasing depth of burial.Thus, if a given formation lies about 10,000 feet beneath the surface,it may be expected to have, on the average, a water content of about 2weight percent. It is then possible to use this water content, incombination with the method described earlier for determining the vaporpressure of a formation within the earth, to arrive at an approximatevalue of the vapor pressure possessed by the formation in its naturalcondition within the earth.

3. Drilling Fluid Design Once the formation vapor pressure is known, itis then possible to select and formulate a water-in-oil drilling fluidhaving an aqueous phase vapor pressure which bears a particular relationto the aqueous vapor pressure of the formation. Generally speaking, itis desirable that the aqueous phase of the drilling fluid have anaqueous vapor pressure no greater than that of the water-sensitiveformation. This frequently requires the aqueous vapor pressure of thedrilling fluid to be less than that of a saturated sodium chloridesolution and often it is desirable to saturate the aqueous phase of thedrilling fluid with calcium chloride. As pointed out above with respectto drilling fluid design by the simulation method, it is especiallydesirable to maintain the aqueous activity of the mud at a level aboutequal to that of the water-sensitive formation. Balancing the activitiesin this fashion eliminates any substantial migration of water betweenthe emulsion fluid and the formation, thereby eliminating any sloughingof the borehole as well as contamination of the mud by water containedwithin the shale.

However, economics or other considerations may occasionally make itundesirable to attempt to completely reduce the aqueous vapor pressureof the drilling fluid to that of the formation. As long as the watertransfer between the fluid and formation is insufficient to causeexcessive formation failure during the time period the water-sensitiveformation is exposed to the wellbore, the aqueous vapor pressure of thedrilling fluid can be considered to be substantially equal to that ofthe water-sensitive formation. However, it is preferable to reducedrilling fluid vapor pressure to a level that is equal to or below thatof the formation.

It should be noted that mixtures of salts can be used in the water phaseof an invert, but such mixtures are subject to the common-ion effect.Their aqueous solutions may thus have higher aqueous vapor pressuresthan would otherwise be suggested by the total salt concentration. Itshould also be noted that the emulsifier and other water-solubleconstituents of the drilling fluid may tend to slightly alter the vaporpressure of the aqueous solution containing the vapor pressuredepressants when the emulsion fluid is prepared. Thus the aqueous vaporpressure of the emulsion fluid, which is the aqueous vapor pressure ofthe water phase of the emulsion fluid, may differ slightly from that ofthe aqueous salt solution used to prepare the emulsion. Generallyspeaking, however, an invert emulsion drilling fluid wherein the aqueousphase is saturated with sodium chloride may be used where the vaporpressure of the formation has a value (P) about threefourths of thevapor pressure of water (P,,) at the same temperature (i.e., a relativevapor pressure of 0.75). Referring to FIG. 8, such a mud would besuccessful in drilling the deep, hard, West Texas shale (A) shown therewhich possesses a natural water content of about 2.2 weight percent. Inthat regard, it should be noted that hard argillaceous shales seldomexhibit relative aqueous vapor pressures in excess of 0.75. The deep,hard, Louisiana shale (B), for example, would normally be drilled withan invert emulsion fluid wherein the aqueous phase consists of saturatedcalcium chloride solution (i.e., a relative vapor pressure of 0.30).This shale has a connate or natural water content of about 1.8 or 1.9weight percent. The soft, gumbo shale (C) has a connate or natural watercontent of about ll weight percent and is best satisfied by a fluid withan aqueous phase vapor pressure less than a saturated aqueous NaClsolution.

C. Emulsion Drilling Fluids Designed by the Methods of the InventionReferring specifically to water-in-oil invert emulsion drilling fluids,a variety of such fluids are commercially available for use in drillingwells. Fluids of this type may be modified by the addition of vaporpressure depressants and can be used for drilling through watersensitiveformations difficult to drill with the commercial fluids. Typical invertemulsion drilling fluids contain droplets of water finely dispersed oremulsified in an oil base. Diesel fuels, kerosenes, and high-gravitycrude oils are frequently used as the oil base; and about 10 to 70percent of fresh or common salt water is emulsified therein with thehelp of suitable emulsifying and stabilizing agents. Anionic, nonionic,and mixed anionic-nonionic emulsifiers are all used for this purpose.The emulsifiers and stabilizing agents employed in the fluids should becompatible with sodium chloride, calcium chloride, or whatever watervapor pressure depressant is to be incorporated in the aqueous phase ofthe modified compositions. One specific invert emulsion drilling fluidcomposition which has been tested and appears satisfactory for manyapplications comprises 70 volume percent No. 2 diesel fuel; 25 volumepercent water saturated with calcium chloride; and 5 volume percentsorbitan mono-oleate as the emulsifier. No difficulty, however, has beenencountered in obtaining other satisfactory compositions simply byadding sodium chloride or calcium chloride to certain existingcommercially available invert emulsion drilling fluids. Where formationsare particularly water-sensitive it may be desirable to prepare adrilling fluid having a vapor pressure which is less than that of a mudcontaining a saturated calcium chloride solution. Solutions containingZnBr ZnC1 LiBr, LiCl, or similar water-soluble saltscan be employed forthis purpose. In addition, it has been found that a supersaturated CaClmud can be formed by adding additional CaCl to a mud having a saturatedCaCl solution as the aqueous phase. Such supersaturated CaCl muds havevapor pressures lower than those of saturated CaCl muds. Other watervapor depressants contemplated to be useful in the various embodimentsof this invention include still other water-soluble salts; phosphoricacid, acetic acid, and other water-soluble acids; glycerol; sodiumhydroxide; potassium hydroxide; etc.

D. Monitoring the Drilling Fluid at the Wellsite Once a compatibledrilling fluid has been selected and introduced into the drillingsystem, it is advisable to monitor the fluid periodically to insureretention of compatibility. Contaminants, absorption, and otherphenomena may cause gradual changes in the composition of the mud.Monitoring can be rapidly accomplished by periodically immersing samplesof successive formations penetrated by the well in portions of the mudin contact with these formations and logging the direction and extent ofwater migration between each such sample and the mud in which it isimmersed with the displacement transducer apparatus.

In some cases it may be desirable to monitor the mud with calibratedshale samples having known vapor pressures. These calibrated samples maybe preserved samples of the formation being drilled that have been takenfrom another well. Synthetic shale specimens, clay specimens, and thelike, prepared so that they have particular aqueous vapor pressures canalso be used. Calibrated shale samples representative of thewatersensitive formation are equivalent to substantially unalteredformation samples and the necessity for obtaining such samples from thewell being drilled can thus be eliminated. By comparing an oil-basefluid of unknown aqueous vapor pressure with shale samples having knownaqueous vapor pressures, it is apparent that the vapor pressure of thefluid can be determined. In this connection, it may be desireable tocontinuously monitor the aqueous vapor pressure of the oil-base mud withthe displacement transducer and compare it with the formation vaporpressure. Another convenient method to monitor the aqueous vaporpressure of the mud is to place a sample in a closed container anddirectly measure the relative humidity of the atmosphere in contact withthe samples as is discussed above.

The condition and composition of the oil-base fluid can be determined byperiodically emulsion-breaking a mud sample and determining its watercontent. In addition, the water can be analyzed for its content of vaporpressure depressant. Thus, if the vapor pressure of the aqueous phase isbeing controlled by the presence of calcium chloride, the aqueous phasecan be analyzed for this salt.

The condition of the drilling fluid can also be qualitatively evaluatedby observing the cuttings produced in the drilling operation if thewater-sensitive formation is such that it will undergo visibledeformation as it absorbs water. If the cuttings are firm and uniform,it can therefore be inferred that the fluid and the formation are insatisfactory condition. On the other hand, if the cuttings become softeror more diffuse, the concentration of the vapor pressure depressant inthe aqueous phase of the fluid should be increased.

If an invert emulsion drilling fluid prepared in accordance with theinvention loses water from its aq ueous phase during drilling, it isprobable that the water is being absorbed by the surrounding formation;and if this is the case, drilling conditions will tend to become moreadverse. It is therefore desirable, under such circumstances, to addvapor pressure depressant to the aqueous phase of the fluid until itsaqueous vapor pressure is no greater than the aqueous vapor pressure ofthe formation being drilled. This can generally be done by vigorouslymixing the fluid at the surface of the earth with fresh depressant.

As noted previously, drilling fluids prepared in accordance with thepresent inventionare especially applicable for use in the drilling ofhard shales. Until the advent of this invention, there has been nosatisfactory procedure for dealing with such shales. As notedpreviously, such hard shales generally have aqueous activities less than0.75 which corresponds to a saturated solution of NaCl. The resultsobtained in accordance with the invention have shown that invertemulsion muds wherein the aqueous phase is water saturated with calciumchloride are remarkably effective for a wide variety of such shales. If,during the course of drilling such a shale, additional calcium chloridemust be added to the mud system, this may be done by mixing powderedcalcium chloride into the fluid. Powdered calcium chloride has beenfound to readily enter the aqueous phase of an invert emulsion drillingfluid.

Abnormal pressure zones represent serious drilling hazards in many areaswhere wells are drilled. One characteristic of such zones is atransition zone that lies just above the abnormal pressure zone and thatexhibits a marked increase in water content. Since a correspondingincrease may also be observed in the water activity of shales in thetransition zone, continuously logging the activity of formationspenetrated provides a method of detecting abnormal pressure zones. Wateractivity of a shale is reflected by the ratio of its aqueous vaporpressure to the vapor pressure of pure water at the same temperature,i.e., relative humidity. It may therefore be desirable to log theaqueous vapor pressure of the drilling cuttings of the formations asthey are penetrated. Measurements can be performed by exposing thecuttings to atmospheres of varying known humidities as discussed above,by placing the cuttings in a closed container and directly measuring therelative humidity of the atmosphere in contact with them as alsodiscussed above, or by using the displacement transducer apparatus inconjunction with a series of oilbase fluids having known aqueous vaporpressures. If the aqueous vapor pressure of the shale is equal to thatof the oil-base fluid, when the shale sample is placed in contact withthe fluid it will exhibit no deformation.

II. Used of the Methods of the Invention for Other Fluids A. TreatingFluids The principles of this invention are also applicable to otherwiseconventional well fluid compositions such as packer fluids, coringfluids, completion fluids, and well treating fluids. With respect totreating fluids, for example, the methods and apparatus of the inventionare useful in designing fluids for repairing and restoring water-damagedformations. In the past, it has been conventional practice in the fieldto attempt to restore water-damaged formations by treating them withconcentrated salt water (30,00050,000 ppm) or with so]- vents such asalcohols which have at least some degree of miscibility with both waterand hydrocarbons. In accordance with the present invention, a suitabletreating fluid is a water-in-oil emulsion wherein the aqueous phase hasa sufficiently low vapor pressure so as to attract water from thedamaged formation, thereby dehydrating and restoring the formation.Suitable oils for use in the emulsion include diesel fuels, kerosenes,light fuel oils, light crude oils, light petroleum fractions, LPGs, andthe like. Oil-base and water-in-oil invert emulsion drilling fluidscontaining vapor pressure depressants are also generally suitable foruse as packer fluids, coring fluids, completion fluids, etc.

B. Displacement Fluids The principles of the invention are alsoapplicable to fluid compositions and methods used in displacing oil fromreservoirs. In recent years, for example, it has been observed thatwater-in-oil emulsions and microemulsions are useful in displacing oilfrom reservoirs. Such fluids generally are prepared from the same typesof oils used to prepare invert emulsion treating fluids. Soluble oilshave been employed to form microemulsion displacement fluids. Suchformulations are typified by the displacement fluids disclosed in US.Pat. No. 3,254,714. The emulsion or microemulsion is injected into areservoir at one point and driven from that point through the reservoirtoward a second point where displaced oil is recovered from thereservoir. Since such emulsions and microemulsions have a substantialdegree of miscibility with reservoir oils, and since their viscositiescan be controlled to a considerable degree, they appear attractive foruse as oil-displacing media. If such fluids, however, are employed informations which are shaley or have shale streaks, there is a tendencyfor the shales to interfere with the effectiveness and stability of theemulsions. This tendency can be reduced through application of thepresent invention by controlling the vapor pressure of the aqueous phaseof the emulsions or microemulsions so that it is substantially equal toor less than the aqueous vapor pressure of the shaley constituents ofthe formation. The manner of control is the same as that described fordrilling fluids earlier in this disclosure. Since the vapor pressure ofdroplets of a liquid become significantly higher than the vapor pressureof the bulk liquid itself if the droplets are small enough, dropletdiameter can be a design consideration in formulating microemulsiondisplacement fluids. Data on the effect of droplet diameter is presentedby Paul Becher on page 8 of Emulsions: Theory and Practice, ReinholdPublishing Corporation, New York 1957).

C. Fracturing Fluids Many of the hydraulic fracturing fluids used tostim ulate oil wells contain water. When such fracturing fluids are usedin the presence of argillaceous, water-sensitive formations, theformations tend to swell and are thereby damaged. This damage can beprevented by using oil-base or water-in-oil invert emulsion fracturingfluids prepared in accordance with this invention. For fracturing fluidsthe amount of vapor pressure depressant added to the aqueous phase ofthe emulsion fluid should be sufficient to reduce the aqueous vaporpressure of the emulsion fluid to a level substantially equal to that ofthe water-sensitive formation. A particularly successful fracturingmethod which is described in US. Pat. No. 3,378,074 utilizes a viscousdispersion of water-in-oil as a fracturing fluid. The viscous fluid islubricated down the borehole by means of an annular ring of water. Sincethe fracturing fluid and the annular ring are subjected to extremeturbulence as the combined stream is forced through perforations andinto the formation to be fractured, it appears that at least temporarilyboth combine to form a water-in-oil emulsion. As a result, it isdesirable to add a vapor pressure depressant to both the internal phaseof the fracturing fluid and the water used to form the annular ring.

III. General It will be understood, and particularly so with respect tothe claims which follow, that while numerous references are made hereinto the aqueous activity, relative humidity or relative vapor pressure ofmaterials, e.g., earth formations, samples of such formations andwater-containing fluids, in each case the quantitative value referred tois the ratio of the aqueous vapor pressure of the material to the vaporpressure of water at the same temperature. This ratio is proportional tothe aqueous vapor pressure of the material, can be measured rapidly andaccurately and has proved to be a convenient quantitative value forcharacterizing the aqueous vapor pressures of material employed or actedupon in association with the methods of the invention. Along these samelines, it will also be understood that the aqueous vapor pressurerelationships between well fluids and earth formations referred toherein refer to the aqueous vapor pressures of materials as they existat downhole temperatures. Also, when the aqueous vapor pressure of anearth formation is mentioned it is assumed that the earth formation isat its natural state of hydration. A particular advantage incharacterizing the aqueous vapor pressure of a material by its relativeaqueous vapor pressure is the relative insensitivity of the ratio totemperature when compared to the absolute aqueous vapor pressure. Thisis particularly advantageous where measurements are carried out on wellfluids and formation samples in the laboratory or in the field atambient conditions for the purpose of designing emulsion fluids fordownhole conditions. That measurements of relative aqueous vaporpressure conducted at atmospheric conditions of temperature and pressureare very good approximations of downhole conditions has beendemonstrated repeatedly by the excellent results achieved when usingfluids designed by these techniques in actual well drilling operations.

What is claimed is:

1. A method of drilling a well in a water-sensitive, argillaceousformation with a water-in-oil emulsion drilling fluid which comprisesmonitoring the aqueous vapor pressure of said drillingfluid during thedrilling operation and maintaining an aqueous vapor pressure depressantin the aqueous phase of said drilling fluid in .a concentrationsufficient to substantially prevent migration of water from said fluidto said formation.

2. A method as defined in claim 1 in which the aqueous vapor pressure ofthe drilling fluid is monitored by measuring the relative humidity ofthe atmosphere in contact with a sample of the drilling fluid.

3. A method as defined in claim 1 in which said water-sensitiveformation is an argillaceous hard shale formation.

4. A method as defined in claim 1 in which a sufficient concentration ofsaid aqueous vapor pressure depressant is maintained in the aqueousphase of said drilling fluid to render the aqueous vapor pressure of thedrilling fluid about equal to the aqueous vapor pressure of saidwater-sensitive formation,

5. A method as defined in claim 4 in which said water-sensitiveformation is an argillaceous hard shale formation.

6. In a well operation in which a water-sensitive, argillaceous shaleformation having a natural state of hydration is contacted with awater-in-oil emulsion fluid, the improvement which comprises maintainingthe aqueous vapor pressure of said emulsion fluid at a value which willprevent a sample of said formation at said natural state of hydrationfrom absorbing water from said emulsion fluid.

7. A method as defined in claim 6 in which the earth formation is a hardshale.

8. A method as defined in claim 6 in which the aqueous vapor pressure ofsaid emulsion fluid is maintained at a value which will preventmigration of water between said sample and said emulsion fluid.

9. A method of drilling a well through a subterranean, water-sensitive,argillaceous formation comprising determining the aqueous vapor pressureof a sample of said formation having substantially the same state ofhydration as the natural hydration of said formation and drilling saidformation with a water'in-oil emulsion drilling fluid having an aqueousvapor pressure substantially no greater than the aqueous vapor pressureof said sample.

10. A method as defined in claim 9 in which the aqueous vapor pressureof said sample is determined by measuring the relative humidity of theatmosphere in contact with the sample.

11. A method as defined in claim 10 in which said atmosphere in contactwith said sample is in substantial equilibrium with said sample.

12. In a method of drilling a well in a water-sensitive, argillaceousformation with a water-in-oil emulsion drilling fluid containing anaqueous vapor pressure depressant within the aqueous phase, theimprovement which comprises measuring the degrees of attractionpossessed by one or more samples of said formation for water in saidemulsion fluid upon contact therewith and at different sample watercontents, determining the concentration of aqueous vapor pressuredepressant within said aqueous phase required to substantially preventmigration of water from said aqueous phase to a sample of said formationfor a water content substantially equal to that of said formation, anddrilling said formation with said emulsion fluid containing saidrequired concentration of vapor pressure depressant in the aqueous phasethereof.

13. In a method of drilling a well in a water-sensitive, argillaceousshale formation with a water-in-oil emulsion drilling fluid wherein theaqueous vapor pressure of said drilling fluid is greater than theaqueous vapor pressure of said formation during drilling, theimprovement which comprises adding a water soluble, aqueous vaporpressure depressant to said drilling fluid in a quantity sufficient tolower the aqueous vapor pressure of said drilling fluid to a valuesubstantially equal to the aqueous vapor pressure of said formation.

14. A method as defined in claim 13 in which said water-sensitiveformation is an argillaceous hard shale formation.

15. In a method of drilling a well in a water-sensitive, argillaceousshale formation with a water-in-oil emulsion drilling fluid wherein theaqueous vapor pressure of said drilling fluid during drilling is lessthan the aqueous vapor pressure of said formation, the improvement whichcomprises increasing the aqueous vapor pressure of said drilling fluidto a value substantially equal to the aqueous vapor pressure of saidformation.

16. A method as defined in claim 15 in which the aqueous vapor pressureof the drilling fluid is increased by adding water to the aqueous phaseof said drilling fluid.

17. In a method of drilling a well in a water-sensitive, argillaceousshale formation with a water-in-oil emulsion drilling fluid, theimprovement which comprises maintaining an aqueous vapor pressuredepressant in the water phase of the drilling fluid in a concentrationsufiicient to prevent the absorption of water by said water-sensitiveformation from said drilling fluid.

18. A method as defined in claim 17 in which said water-sensitive shaleformation is a hard, argillaceous shale formation.

19. In a method of drilling a well through a watersensitive,argillaceous shale formation with a water-inoil emulsion drilling fluid,the improvement which comprises maintaining the aqueous vapor pressureof said emulsion fluid at a value which will prevent the migration ofwater from said emulsion fluid to said formation.

20. A method as defined in claim 19 in which said formation is a hardshale.

21. A method as defined by claim 19 in which the aqueous vapor pressureof said emulsion fluid is maintained at a value which will preventmigration of water between said emulsion fluid and said formation.

22. A method as defined in claim 21 in which said formation is a hardshale.

23. In a method of drilling a well in a hard, argillaceous shaleformation with a water-in-oil emulsion drilling fluid containing anaqueous vapor pressure depressant in the aqueous phase thereof, theimprovement which comprises maintaining the aqueous vapor pressure ofthe drilling fluid at a level that is substantially no greater than theaqueous vapor pressure of said hard argillaceous shale formation.

24. In a method of circulating a water-in-oil emulsion fluid within awell in a water-sensitive, argillaceous shale formation having anaqueous activity less than 0.75, the improvement which comprisesmaintaining an aqueous vapor pressure depressant in the water phase ofsaid fluid in a concentration such that the aqueous activity of saidfluid is substantially equal to the aqueous activity of said formation.

25. A method as defined by claim 24 wherein the aqueous vapor pressuredepressant is calcium chloride and the aqueous activity of saidformation is not substantially lower than 0.3.

26. In a method of drilling a well through a hard, argillaceous shaleformation having an aqueous activity less than 0.75, the improvementwhich comprises circulating a water-in-oil emulsion drilling fluidwithin said well, said drilling fluid having an aqueous activity aboutequal to that of said formation.

27. A method as defined in claim 26 wherein the aqueous activity of saidshale formation is not substantially lower than 0.3 and the aqueousphase of said drilling fluid is an aqueous solution of calcium chloride.

28. In a method of drilling a well through a watersensitive shaleformation with a water-in-oil emulsion drilling fluid, the improvementwhich comprises determining the type and concentration of an aqueousvapor pressure depressant required in the aqueous phase of said emulsionfluid to prevent said formation from withdrawing water from saidemulsion fluid and maintaining the concentration of said aqueous vaporpressure depressant in the aqueous phase of said emulsion fluid at alevel substantially no lower than said required concentration.

29. The method of claim 28 wherein said aqueous vapor pressuredepressant is a water-soluble salt.

30. The method of claim 29 wherein said aqueous vapor pressuredepressant is sodium chloride.

31. The method of claim 29 wherein said aqueous vapor pressuredepressant is calcium chloride.

32. In a method of drilling a well through a watersensitive shaleformation with a water-in-oil emulsion drilling fluid, the improvementwhich comprises determining the type and concentration of an aqueousvapor pressure depressant required in the aqueous phase of said emulsionfluid to prevent migration of water between said emulsion fluid and saidformation and maintaining the concentration of said aqueous vaporpressure depressant in the aqueous phase of said emulsion fluid at alevel substantially no lower than said required concentration.

33. The method of claim 32 wherein said aqueous vapor pressuredepressant is a water-soluble salt.

34. The method of claim 33 wherein said aqueous r ress re de ressantissdium chloride. vagg. The i netho of claim 3 wherein said aqueous vaporpressure depressant is calcium chloride.

2. A method as defined in claim 1 in which the aqueous vapor pressure of the drilling fluid is monitored by measuring the relative humidity of the atmosphere in contact with a sample of the drilling fluid.
 3. A method as defined in claim 1 in which said water-sensitive formation is an argillaceous hard shale formation.
 4. A method as defined in claim 1 in which a sufficient concentration of said aqueous vapor pressure depressant is maintained in the aqueous phase of said drilling fluid to render the aqueous vapor pressure of the drilling fluid about equal to the aqueous vapor pressure of said water-sensitive formation,
 5. A method as defined in claim 4 in which said water-sensitive formation is an argillaceous hard shale formation.
 6. In a well operation in which a water-sensitive, argillaceous shale formation having a natural state of hydration is contacted with a water-in-oil emulsion fluid, the improvement which comprises maintaining the aqueous vapor pressure of said emulsion fluid at a value which will prevent a sample of said formation at said natural state of hydration from absorbing water from said emulsion fluid.
 7. A method as defined in claim 6 in which the earth formation is a hard shale.
 8. A method as defined in claim 6 in which the aqueous vapor pressure of said emulsion fluid is maintained at a value which will prevent migration of water between said sample and said emulsion fluid.
 9. A method of drilling a well through a subterranean, water-sensitive, argillaceous formation comprising determining the aqueous vapor pressure of a sample of said formation having substantially the same state of hydration as the natural hydration of said formation and drilling said formation with a water-in-oil emulsion drilling fluid having an aqueous vapor pressure substantially no greater than the aqueous vapor pressure of said sample.
 10. A method as defined in claim 9 in which the aqueous vapor pressure of said sample is determined by measuring the relative humidity of the atmosphere in contact with the sample.
 11. A method as defined in claim 10 in which said atmosphere in contact with said sample is in substantial equilibrium with said sample.
 12. In a method of drilling a well in a water-sensitive, argillaceous formation with a water-in-oil emulsion drilling fluid containing an aqueous vapor pressure depressant within the aqueous phase, the improvement which comprises measuring the degrees of attraction possessed by one or more samples of said formation for water in said emulsion fluid upon contact therewith and at different sample water contents, determining the concentration of aqueous vapor pressure depressant within said aqueous phase required to substantially prevent migration of water from said aqueous phase to a sample of said formation for a water content substantially equal to that of said formation, and drilling said formation with said emulsion fluid containing said required concentration of vapor pressure depressant in the aqueous phase thereof.
 13. In a method of drilling a well in a water-sensitive, argillaceous shale formation with a water-in-oil emulsion drilling fluid wherein the aqueous vapor pressure of said drilling fluid is greater than the aqueous vapor pressure of said formation during drilling, the improvement which comprises adding a water soluble, aqueous vapor pressure depressant to said drilling fluid in a quantity sufficient to lower the aqueous vapor pressure of said drilling fluid to a value substantially equal to the aqueous vapor pressure of said formation.
 14. A method as defined in claim 13 in which said water-sensitive formation is an argillaceous hard shale formation.
 15. In a method of drilling a well in a water-sensitive, argillaceous shale formation with a water-in-oil emulsion drilling fluid wherein the aqueous vapor pressure of said drilling fluid during drilling is less than the aqueous vapor pressure of said formation, the improvement which comprises increasing the aqueous vapor pressure of said drilling fluid to a value substantially equal to the aqueous vapor pressure of said formation.
 16. A method as defined in claim 15 in which the aqueous vapor pressure of the drilling fluid is increased by adding water to the aqueous phase of said drilling fluid.
 17. In a method of drilling a well in a water-sensitive, argillaceous shale formation with a water-in-oil emulsion drilling fluid, the improvement which comprises maintaining an aqueous vapor pressure depressant in the water phase of the drilling fluid in a concentration sufficient to prevent the absorption of water by said water-sensitive formation from said drilling fluid.
 18. A method as defined in claim 17 in which said water-sensitive shale formation is a hard, argillaceous shale formation.
 19. In a method of drilling a well through a water-sensitive, argillaceous shale formation with a water-in-oil emulsion drilling fluid, the improvement which comprises maintaining the aqueous vapor pressure of said emulsion fluid at a value which will prevent the migration of water from said emulsion fluid to said formation.
 20. A method as defined in claim 19 in which said formation is a hard shale.
 21. A method as defined by claim 19 in which the aqueous vapor pressure of said emulsion fluid is maintained at a value which will prevent migration of water between said emulsion fluid and said formation.
 22. A method as defined in claim 21 in which said formation is a hard shale.
 23. In a method of drilling a well in a hard, argillaceous shale formation with a water-in-oil emulsion drilling fluid containing an aqueous vapor pressure depressant in the aqueous phase thereof, the improvement which comprises maintaining the aqueous vapor pressure of the drilling fluid at a level that is substantially no greater than the aqueous vapor pressure of said hard argillaceous shale formation.
 24. In a method of circulating a water-in-oil emulsion fluid within a well in a water-sensitive, argillaceous shale formation having an aqueous activity less than 0.75, the improvement which comprises maintaining an aqueous vapor pressure depressant in the water phase of said fluid in a concentration such that the aqueous activity of said fluid is substantially equal to the aqueous activity of said formation.
 25. A method as defined by claim 24 wherein the aqueous vapor pressure depressant is calcium chloride and the aqueous activity of said formation is not substantially lower than 0.3.
 26. In a method of drilling a well through a hard, argillaceous shale formation having an aqueous activity less than 0.75, the improvement which comprises circulating a water-in-oil emulsion drilling fluid within said well, said drilling fluid having an aqueous activity about equal to that of said formation.
 27. A method as defined in claim 26 wherein the aqueous activity of said shale formation is not substantially lower than 0.3 and the aqueous phase of said drilling fluid is an aqueous solution of calcium chloride.
 28. In a method of drilling a well through a water-sensitive shale formation with a water-in-oil emulsion drilling fluid, the improvement which comprises determining the type and concentration of an aqueous vapor pressure depressant required in the aqueous phase of said emulsion fluid to prevent said formation from withdrawing water from said emulsion fluid and maintaining the concentration of said aqueous vapor pressure depressant in the aqueous phase of said emulsion fluid at a level substantially no lower than said required concentration.
 29. The method of claim 28 wherein said aqueous vapor pressure depressant is a water-soluble salt.
 30. The method of claim 29 wherein said aqueous vapor pressure depressant is sodium chloride.
 31. The method of claim 29 wherein said aqueous vapor pressure depressant is calcium chloride.
 32. In a method of drilling a well through a water-sensitive shale formation with a water-in-oil emulsion drilling fluid, the improvement which comprises determining the type and concentration of an aqueous vapor pressure depressant required in the aqueous phase of said emulsion fluid to prevent migration of water between said emulsion fluid and said formation and maintaining the concentration of said aqueous vapor pressure depressant in the aqueous phase of said emulsion fluid at a level substantially no lower than said required concentration.
 33. The method of claim 32 wherein said aqueous vapor pressure depressant is a water-soluble salt.
 34. The method of claim 33 wherein said aqueous vapor pressure depressant is sodium chloride.
 35. The method of claim 33 wherein said aqueous vapor pressure depressant is calcium chloride. 